Environmental logging system

ABSTRACT

An environmental logging system that includes a housing and an electronic logging circuit sealed within the housing. The logging circuit includes an energy source, transducer, charging circuit, trigger circuit, electronic control unit (ECU), and transmitter. The energy source is charged by the charging circuit using electricity from the transducer in response to external energy applied to the transducer. Commands may also be received by the circuit via the transducer. The ECU includes a processor, memory, and one or more sensors, and it operates under power from the energy source to store data from the sensor(s). The transmitter is coupled to the ECU and transmits the data via electromagnetic radiation outside the housing. The trigger circuit supplies operating power to the processor only when the voltage level of the energy source is above a minimum threshold. The housing may be a tubular shell filled with a polymeric material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/280,353 filed Jan. 19, 2016, the entire contents ofwhich are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to small scale recording devices for loggingenvironmental conditions and which can be used in fluids andenvironments that may involve a significant range of temperatures andpressures.

BACKGROUND

Environmental logging microsystems (ELMs) of the type described hereinmay be deployed into environments where they will sense and store datathat can thereafter be retrieved for analysis and reporting. One suchapplication of the ELMs is in oilfield wellbores where the ELMs may besubjected to high temperatures and pressures.

SUMMARY

In accordance with an aspect of the invention, there is provided anenvironmental logging system that includes a housing and an electroniclogging circuit located within the housing. The logging circuit includesan energy source, transducer, charging circuit, electronic control unit(ECU), and a trigger circuit. The energy source supplies electricity topower the logging circuit. The transducer that converts external energyapplied to the transducer into electricity, and the charging circuitcouples the transducer to the energy source such that the transducercharges the energy source when the transducer is subjected to theexternal energy. The ECU operates from the electricity supplied by theenergy source and includes a processor, non-volatile memory, and one ormore sensors, each of which detects an environmental condition andprovides a sensor signal indicative of a value of the environmentalcondition. The ECU operates under control of a program to store in thenon-volatile memory data representative of the sensor signal(s). Thetrigger circuit operates from the electricity supplied by the energysource and is coupled to the ECU. The energy source provides theelectricity at different voltage levels depending on a state of chargeof the energy source and the processor is rated for normal operation atvoltages above a minimum operating voltage and draws more power at somevoltages under the minimum operating voltage. The trigger circuitinhibits power draw by the processor at voltages under the minimumoperating voltage. This helps avoid wasted power consumption by theprocessor when the state of charge of the energy source is below theminimum operating voltage of the processor.

In accordance with another aspect of the invention, there is provided anenvironmental logging system that includes a housing and an electroniclogging circuit carried by the housing. The logging circuit includes anenergy source, transducer, charging circuit, ECU, and transmitter. Thecharging circuit interconnects the energy source and transducer, and theenergy source is charged by the charging circuit using electricity fromthe transducer in response to external energy applied to the transducer.The ECU includes a processor, memory, and one or more sensors, whereinthe ECU operates under power from the energy source to store data fromthe sensor(s). The transmitter is coupled to the ECU and transmitselectromagnetic radiation outside the housing. The ECU is coupled to thetransducer and is configured to detect electricity from the transducerthat is indicative of pulses of external energy impinging on thetransducer. The ECU is configured to respond to one or more commandsencoded by the pulses to change operating states between a sleep state,detection state, and readout state. And the ECU is configured to recorddata from the one or more sensors when in the detection state and tosend data via the transmitter when in the readout state.

In accordance with yet another aspect of the invention, there isprovided an environmental logging system that includes a housing and anelectronic logging circuit carried by the housing. The logging circuitincludes an energy source, transducer, charging circuit, and ECU. Thecharging circuit interconnects the energy source and transducer, and theenergy source is charged by the charging circuit using electricity fromthe transducer in response to external energy applied to the transducer.The ECU includes a processor, memory, and one or more sensors, whereinthe ECU operates under power from the energy source to store data fromthe sensor(s). The housing comprises a tubular shell filled with apolymeric material that seals the logging circuit within the shell.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments will hereinafter be described inconjunction with the appended drawings, wherein like designations denotelike elements, and wherein:

FIG. 1 is a block diagram of a first embodiment of an environmentallogging microsystem (ELM) constructed in accordance with the presentinvention;

FIG. 2 is a block diagram of a second embodiment of an ELM constructedin accordance with the present invention;

FIG. 3 is a schematic diagram of an electronic logging circuit used inthe ELM of FIG. 1;

FIG. 4 is a graph showing measured current consumption of the processorused in the logging circuit of FIG. 3, as a function of its supplyvoltage;

FIGS. 5 and 6 diagrammatically illustrate an exemplary physicalcomponent layout of the logging circuit of FIG. 3 on a printed circuitboard in both a flat (FIG. 5) and folded (FIG. 6) condition;

FIGS. 7A and 7B are images of actual prototypes showing the manufacturedlogging circuit of FIGS. 3 and 5-6 prior to folding of the printedcircuit board (FIG. 7A) and after folding when incorporated into ahousing (FIG. 7B);

FIG. 8 is a diagram of a software program carried out by the loggingcircuit of FIG. 3, showing its operation as a state machine with thedifferent operating states used to carry out the functions of thecircuit;

FIG. 9 is a diagram showing a set of sub-states of a readout state ofthe program of FIG. 8;

FIG. 10 is a diagram showing a set of sub-states of a detection state ofthe program of FIG. 8;

FIG. 11 is a flow chart of an exemplary field test for temperaturelogging using one of the illustrated ELMs;

FIG. 12 depicts graphs of temperature data recorded by prototype ELMsduring stress testing;

FIG. 13 is a schematic diagram of an electronic logging circuit used inthe ELM of FIG. 2;

FIG. 14 is a state diagram of the operational states carried out by thesoftware program of the ELM of FIG. 2;

FIGS. 15 and 16 show an exemplary printed circuit board layout for thelogging circuit of FIG. 13 along with tri-fold stacking of thecomponents for packaging of the logging circuit into a housing;

FIGS. 17 and 18 illustrate different embodiments of packaging designsfor the illustrated ELMs that utilize a tubular shell to house thecircuit components;

FIGS. 19A-19D show different tubular shells that may be used forhousings of the illustrated ELMs;

FIG. 20 is an image of an actual prototype showing a manufacturedlogging circuit such as in FIG. 13 or 21 using the printed circuit boardlayout of FIGS. 15-16 in a tubular housing such as shown in FIG. 19B;

FIG. 21 is a schematic diagram of an electronic logging circuit used ina third embodiment of an ELM constructed in accordance with the presentinvention; and

FIG. 22 is an image of an actual prototype showing the manufacturedlogging circuit of FIG. 21 as it would be utilized in a flat (unfolded)condition.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Shown in FIG. 1 is an embodiment of an electronic logging microsystem(ELM), designated generally as ELM 20. ELM 20 includes a housing 22 withan electronic logging circuit in the form of an ELM circuit 24 that ishermetically sealed within the housing 22. An external light source 26provides charging energy to the ELM 20 as well as encoded commands thatcause ELM 20 to switch between various states, including a sleep state,detection state, and readout state. An external optical receiver 28 isused to read light pulses emitted from the ELM 20 for readout of storeddata. These and other features will be described below.

At least a portion of housing 22 is transmissive to electromagneticradiation which allows energy from the light source 26 to be received bythe ELM circuit 24. For example, the housing 22 may include an opticallytransmissive lid 23 that functions as an optical window for wirelesscharging by the light source 26. This also allows signaling light fromthe ELM circuit 24 to be transmitted out of the housing for purposes ofreadout by the receiver 28. The ELM circuit 24 includes an energy sourcesuch as a battery 30, a transducer such as a solar cell 32 that convertsthe received light from light source 26 into electricity, a chargingcircuit 34 connected to the solar cell 32 for recharging the battery 30,an electronic control unit (ECU) 36 that is used for recording andreadout of environmental data, a trigger circuit 38 that controls theenergizing of the ECU 36 based on the state of charge of battery 30, andone or more electromagnetic radiation transmitters such as LEDs 40 thatprovide detectable light out of the housing 22 for receipt by theoptical receiver 28.

ECU 36 may include a processor and/or non-volatile memory (not shown) aswell as one or more sensors 42, each of which detects an environmentalcondition (e.g., temperature, pressure, etc.) and provides a sensorsignal indicative of a value of the environmental condition. The ECU 36may operate under control of a control program stored in memory (e.g.,in the non-volatile memory) to receive and store in the non-volatilememory data representative of the sensor signals. In the exemplaryembodiment shown in FIG. 1, a temperature sensor 42 is utilized and isincluded within a low power microcontroller unit (MCU) that comprisesthe ECU 36. In other embodiments, such as shown in FIG. 2, the ECU maycomprise a processor with memory, such as the same MCU as in FIG. 1, aswell as additional sensors for other readings such as pressure andacceleration. In other embodiments, the processor can be a componentthat is separate from any of the sensors or even separate from thememory used for storing the control program and sensor data.

FIG. 2 depicts an ELM 50 which can be implemented using the same orsimilar circuits to that of FIG. 1, but may include other components andcapabilities. Thus, the various aspects of the construction andoperation of ELM 20 discussed herein apply to ELM 50 as well, exceptwhere specific differences or variations are described. One suchdifference or variation is that ELM 50 has additional sensors includingone or more pressure sensors 74 and an inertial measurement unit (IMU)76 that can provide three-axis acceleration and magnetic compassdirectionality data. It also includes an RFID tag 78 that permits eachsuch ELM to be uniquely identified from other ELMs in use via anexternal RFID reader 80. Details of the construction of ELMs 20 and 50will be described below in greater detail.

Referring now to FIG. 3, there is shown ELM circuit 24. This circuitincludes a charging circuit block that comprises the battery 30, solarcell 32, and charging circuit 34. Battery 30 may be a rechargeablelithium battery (e.g., 3 volts; model no. MS412FE, commerciallyavailable from Seiko Instruments Inc.). Solar cell 32 may be amonolithic photovoltaic cell (e.g., 4 volts output; model no. CPC1822commercially available from Clare™), which is connected to battery 30via a diode 25 and overcurrent protection resistor 27. A capacitor 29connected in parallel with the battery 30 stores additional charge tohelp handle large transient current needs of the ELM circuit 24. ECU 36may comprise a MCU (e.g., model no. C8051F990 commercially availablefrom Silicon Labs), which is a packaged integrated circuit that includesa 25 MHz processor with onboard memory, temperature sensor, an inputcoupled to the solar cell 32 for optical triggering of the ELM, andoutputs connected to the LEDs 40 for data readout.

Operating power is supplied to the ECU 36 via the trigger circuit 38,which may be implemented as a Schmitt trigger that inhibits operatingpower to the ECU unless the battery 30 is at a suitably predeterminedhigh state of charge. As will be discussed below, this prevents the drawof power from the MCU at operating voltages (Vdd) below its minimumrating. Schmitt trigger circuit 38 monitors battery voltage (Vcc)against a reference voltage (V_(ref)) using a comparator (e.g., modelno. TLV3012, commercially available from Texas Instruments) withpositive feedback via voltage dividing resistors (R_(c1), R_(c2),R_(c3)) selected to maintain the output (Vow) grounded unless thebattery voltage (Vcc) is at least at a predetermined threshold voltage(e.g., about 2.0 volts). By inhibiting power draw from the MCU when thebattery voltage (Vcc) is below the minimum operating voltage (Vdd) ofthe MCU, the trigger circuit 38 helps avoid undesirable draining ofbattery 30, thereby extending the time required between recharging ofthe battery 30.

Those skilled in the art will recognize that the above-describedcomponents of ELM circuit 24 may all comprise commercial off-the-shelfcomponents. Furthermore, as will be described below, the design of ELMcircuit 24, including programming of the ECU 36, enables operation ofthe ELM 20 and ELM 50 at temperatures exceeding the individual maximumtemperature ratings of at least some of the components.

Another feature of ELM circuit 24 is the connection of the solar cell 32to an input (V_(sensing)) of the ECU 36 via a sensing resistor(R_(sensing)) 31. With suitable programming of the ECU 36, this permitsexternally-sourced commands to be encoded into light pulses that aredirected by the light source 26 onto the solar cell 32 and then read bythe ECU 36. Thus, the ECU 36 may be configured to detect electricityfrom the solar cell 32 that is indicative of the impinging light pulses,and is configured to respond to one or more commands encoded in thelight pulses to change operating states (e.g., between a sleep state,detection state, readout state, etc.). The ECU 36 may be configured torecord data from the one or more sensors 42 when in the detection stateand to send data via the LEDs 40 when in the readout state. These andother operating states of the ECU 36 will be described in greater detailbelow in connection with specific implementations of ELM 20 and ELM 50.

Referring now also to FIGS. 5-7B, there will be described the design,features, and use of embodiments of ELM 20 and ELM 50, which mayfunction as a temperature logger with data storage and communicationcapabilities. FIGS. 5 and 6 depict the PCB 90 and system componentplacement and folding along a folding line 92 into a compact package.FIGS. 7A and 7B show a prototype assembled circuit both before (FIG. 7A)and after (FIG. 7B) folding along the folding line 92 and placement intothe housing 22. An optically transmissive (e.g., made of sapphire) lid23 (not shown in FIG. 7B) is then hermetically sealed over the openingto form a completed housing 22 having an optical window through whichlight may pass for charging, signaling, and readout of data. Forexample, the ELM circuit 24 may be encapsulated in a stainless steelshell 22 (e.g., an 8 mm package) and an optically transmissive sapphirelid 23 (not shown in FIG. 7B). The MCU used for ECU 36 may beresponsible for functional control, data storage and communication, andsystem power management. The solar cell 32 may be used for wirelesscharging of the battery 30, for optical triggering of the systemfunctions during testing, and for post-deployment interrogation. Thecharging circuit 34 assists the wireless charging of the battery 30using the solar cell 32. The wireless charging occurs when the solarcell 32 is sufficiently exposed to continuous light from an externallight source 26. For optical triggering of different MCU operatingstates, the solar cell 32 is exposed to an external light source 26 thatgenerates predefined light pulse patterns. This results in correspondingvoltage pulses from the solar cell 32. These voltage pulses are pickedup by a comparator embedded within the MCU. As further described withrespect to FIG. 8, different pulse patterns trigger the MCU intodifferent functional states, such as states for temperature sensing,data reporting, etc. The measured temperature data during a testingcycle may be stored in the flash memory integrated in the MCU. This datamay be encoded with a cyclic redundancy check (CRC) algorithm fortransmission error detection. The data and the CRC verification bytesmay be transmitted optically to the exterior receiver unit 28 using theLED indicators 40 controlled by digital output pins of the MCU.

As described below in connection with FIGS. 5-7B, the system can beintegrated using a flexible polyimide PCB 90 and folded into a stack forencapsulation in the 8 mm package. The size of the ELM system stack maybe 6.5×6.3×4.5 mm³. The system may have a sleep mode current consumptionof 5 μA at 25° C., and 12 μA at 125° C. The wireless charging time tocharge the battery from 2.0 V to 3.0 V may be ≈10 hours with a 100 μAcharging current. The storage time after system assembly and before thebattery is depleted can exceed two weeks. Beyond that, the systembattery may be configured to be recharged before the system will respondto any optical triggering. The operating frequency of the system clockmay be 32.768 kHz. The program size in the MCU may be 2 KB.

The charging circuit block 34 may include a Seiko Instruments™ MS412FErechargeable lithium battery 30 for energy storage, a Clare™ CPC1822solar cell 32 for wireless optical charging, a rectifier diode 25 toprevent the reverse flow of the charging current, and a protectionresistor R_(protect) 27 to limit the charging current. Note that theresistor R_(p) is used with the MCU comparator for optical triggering,and is not part of the charging circuit. A voltage is generated when thesolar cell 32 is exposed to external light 26. This results in a currentflowing from the solar cell 32, through the rectifier diode 25 and theprotection resistor R_(protect) 27, into the battery 30 for charging.

The battery 30 may use silicon oxide as its anode and lithium manganesecomposite oxide as its cathode. The Seiko Instruments™ MS412FErechargeable lithium battery has a high capacity (1 mA·h) and a smallform factor (14.8 mm×1.2 mm). The specified operating temperature forthe battery is −20° C. to 60° C. High temperature tests of the batteryhave been performed. According to the testing results, the battery cansupport the system operation at 125° C. for at least two 6 hour cycleswithout evident capacity loss. No leakage or explosions were observed inany testing cycle.

The Clare™ CPC1822 solar cell is a monolithic string of photovoltaicsolar cells. The nominal output voltage of CPC1822 is 4.2 V at anilluminance of 6000 lux. The nominal short-circuit output current is 50μA, and this value can increase up to 200 μA with higher intensitylight. This solar cell 32 has a footprint of 5.8×5 mm². The chargingprotection resistor R_(protect) 27 may be selected as 10 kΩ to limit thecharging current to 100 μA, following manufacturer guidelines for use ofthe battery 30.

A buffer capacitor 29 (e.g., 4.7 μF) is connected in parallel to thebattery 30 to accommodate any transient current need when the systemoperates in the active mode. This includes brief use of the analog todigital converter (ADC), temperature sensor, high power oscillator forinitialization of the MCU, etc.

The Schmitt trigger circuit 38 is used to prevent the MCU from awakeningwhen the battery voltage is low. This circuit aids proper recharging ofthe battery 30. As shown in FIG. 4, the measured current consumption ofthe MCU increases rapidly with the MCU supply voltage (Vdd) when Vdd isbetween 0.7 V and 1.7 V. The maximum current consumption reaches 1521 μAwhen Vdd is 1.6 V. As Vdd increases to 1.7 V, the MCU is fully turned onand the current consumption drops to 77 μA, matching the specificationin the MCU datasheet. The 1.7 V supply value is listed in the datasheetas the turn-on threshold voltage (V_(th)) of the MCU. The currentconsumption for Vdd<V_(th) significantly exceeds the maximum allowedcontinuous discharge current of the battery 30 (100 μA). It is also muchhigher than the maximum current that can be produced by the solar cell32 (≈200 μA). Therefore, recharging would be ineffective if the batteryvoltage is less than V_(th) when no Schmitt trigger circuit 38 is used.

The Schmitt trigger circuit 38 may be designed to add hysteresis to theMCU power supply to prevent the MCU from draining any current while thebattery 30 voltage is below V_(th). The trigger circuit 38 may be placedbetween the power output of the charging circuit 34 and the MCU powerinput (Vdd) pin. It may be implemented with a nanowatt comparator 33(such as a Texas Instruments TLV3012) and three resistors 35 (R_(c1),R_(c2), R_(c3)) to form a positive feedback loop. The TLV3012 comparator33 has a low static power consumption (2.8 μA at 25° C.), smallfootprint (2.4×2 mm²), and fast response time (6 μs). This comparator 33has a 1.2 V internal voltage reference. The three resistors 35, R_(c1),R_(c2), and R_(c3), determine the hysteresis threshold voltage V_(H) andV_(L) (V_(H)>V_(L)). The output of the circuit V_(out), which isconnected to the MCU power input (Vdd), remains at 0 V during batterycharging until the battery voltage reaches V_(H), at which point theSchmitt trigger is turned on. When the battery 30 is discharging,V_(out) goes to 0 V when the battery voltage drops to V_(L). The V_(out)is the same as Vcc when the Schmitt trigger is switched on. The valuesof V_(H) and V_(L) are given by the following equations:

$\begin{matrix}{V_{H} = \frac{1.2 \times \left( {R_{C1} + {R_{C2}\left. R_{C3} \right)}} \right.}{R_{C2}{R_{C3}}}} & (1) \\{V_{L} = \frac{{1.2} \times R_{C3}}{R_{C1}{{R_{C2} + R_{C3}}}}} & (2)\end{matrix}$

Exemplary resistance values may be R_(c1)=1 MΩ, R_(c2)=3.9 MΩ andR_(c3)=1.5 MΩ. The calculated V_(H) may be 2.4 V and V_(L) is 1.9 V.Since V_(L)>1.7 V and V_(H)>V_(L), this value set meets the requirementfor protection. The resistors in the range of MΩ may be selected inorder to keep the system current consumption low.

Thus, it will be appreciated that the trigger circuit 38 in useful inavoiding unnecessary power consumption during low battery conditions. Asindicated above, this undesirable power consumption may occur becausemany energy sources such as battery 30 will provide electricity atdifferent voltage levels depending on a state of charge of the battery,whereas many processors such as the MCU are rated for normal operationat voltages above a minimum operating voltage (e.g., above the turn-onthreshold voltage V_(th)), yet they will draw more power at somevoltages under that minimum operating voltage. The trigger circuit 38inhibits power draw by the MCU at voltages under the minimum operatingvoltage, thereby avoiding the relatively large current draw that couldotherwise occur at MCU supply voltages under V_(th), which is 1.7 voltsfor the MCU used in the exemplary implementation, as indicated in FIG.4.

The model no. C8051F990 MCU used for the ECU 36 includes the followingfeatures that make it suitable for at least some embodiments of thepresent invention. (1) Low power consumption: the power consumption ofthis MCU is 84 μA in the active mode, and 0.8 μA in the sleep mode atroom temperature. (2) Small footprint: the footprint of this MCU is 3×3mm², one of the smallest MCU's available with the functions necessaryfor a temperature logger. (3) Integrated temperature sensor: the MCU hasan internal temperature sensor that can sense the environmentaltemperature with 10 bit resolution. This MCU has an 8 KB flash memoryfor programs and data storage. It will be appreciated by those skilledin the art that other ECU processors and circuits may be used as desiredor necessary for other embodiments.

After a system is assembled and packaged, optical communication may beutilized to interact with the MCU. As mentioned earlier, the output ofthe solar cell 32 is fed into the high-impedance comparator input pin(MCU P1.0) of the MCU for this purpose. As will be further explainedwith respect to FIG. 8, the MCU stays in the sleep mode until it isawakened into the active mode and triggered into different operationstates by voltage pulses from the solar cell 32. Temperaturemeasurements are made using the internal temperature sensor. Themeasured temperature data are stored in the flash memory and areoptically transmitted through two LED indicators 40 when requested bythe exterior receiver unit 28. A protection resistor 37 of 50 kΩ may beused with each LED indicator 40 to moderate adequate current from theMCU digital output to the optical indicator for data transmission.

Two resistors may be utilized for the MCU comparator to work with thesolar cell 32 for optical triggering: a parallel resistor R_(p) 39(e.g., 1 MΩ) connected across the positive and ground pins of the solarcell 32, and a series resistor R_(sensing) 31 (e.g., 1 MΩ) connectedbetween the solar cell positive pin and MCU pin P1.0. The R_(p) resistor39 is used to form a current flow path between the two pins of the solarcell 32, thus generating a voltage drop that can be picked up by the MCUcomparator when the solar cell 32 is exposed to light. The R_(sensing)resistor 31 is used to limit the current consumption into MCU P1.0. Whenthe Schmitt trigger is off, the MCU is not powered on, and a modestresistance exists between the MCU P1.0 and ground instead of the highimpedance when the MCU is on. Without the resistor R_(sensing) 31, thispath would draw most of the current generated by the solar cell 32 anddisable the wireless charging capability when the Schmitt trigger isoff. The use of the resistor R_(sensing) 31 limits the current to lessthan 10 μA when the Schmitt trigger is off, and does not affect theoperation of the MCU comparator pin when the trigger is on.

With reference again to FIGS. 5-7B, the ELM circuit 24 components can beintegrated on a flexible polyimide PCB 90 and then folded into a stackfor encapsulation into the 8 mm package. The PCB 90 includes top andbottom metal layers. The polyimide used for the PCB can be DuPont™Pyralux AP, which has a maximum elongation of >50% after curing. All theelectronic components except the battery 30 may be assembled on the toplayer of the PCB 90. FIG. 6 shows a configuration of the system stackafter the polyimide PCB is folded along the folding line 92. The stackhas three layers: the solar cell 32 and LEDs 40 on the top layer, thebattery 30 sandwiched in the middle layer, and the MCU and othercomponents on the bottom layer. The thickness of the polyimide PCB 90may be 300 μm in the component area and 200 μm in the fold. The thickercomponent area can stabilize the components and prevent the solderedpins from breaking during the folding process. The overall stack sizemay be 6.5×6.3×4.5 mm³ (L×W×H).

The ELM 20, 50 system software may be designed for the MCU to providefunctions including system control and power management, temperaturemeasurement, temperature data processing and storage, and bidirectionalcommunication with an exterior unit through the optical link. Asmentioned above, the solar cell 32 in the system can generate differentvoltage pulses according to the light pulse patterns received from anexterior light source. These voltage pulses trigger the MCU intodifferent functional states. As shown in FIG. 8, there are 10 functionalstates and one error state for the system. Two tasks of the system,temperature data collection and reporting, are implemented in thedetection (State 9) and readout (State 8) states, respectively. Otherstates may be used for power management, wireless charging, systemstorage, and optical triggering, etc. The details of each state areprovided below.

State 1, MCU Off State.

This is the state when the battery 30 is not being charged, and itsvoltage (V_(cc)) is below the threshold voltage of the Schimitt trigger(V_(th), which represents either V_(H) during charging or V_(L)otherwise). The system may enter this state when the battery 30 has beendrained below V_(th) (e.g., Vdd<V_(th)), either during storage or afteroperations. The MCU remains off in this state. Any measured data will beretained in the MCU flash memory without loss.

State 2, MCU Off State (Charging).

This is a transitional state in which the battery 30 has a voltage belowV_(th) and is being charged. This state may be skipped if a strongexternal light is illuminating the solar cell 32 to provide enoughcharging current for the battery voltage to rise immediately aboveV_(th).

State 3, MCU Initialization. This is a transient state during which theMCU turns on (e.g., in response to Vdd>V_(th)) and all global constantsand variables are initialized or restored from the flash memory.Registers of pertinent MCU peripherals, e.g., ADC, comparator, digitalI/O, etc., may be initialized for use in later operations.

State 4, Deep Sleep State (No Charging).

This is the state when the MCU is on but in the deep sleep mode tominimize power consumption. The system may enter this state from State 3after finishing the initialization, or after finishing data collectionor reporting operations. This is also the state in which the packagedsystem is stored before deployment and before the battery is depleted.The sensor's red LED may flash twice when this state is entered.

State 5, Pulse Count State.

This is a transitional state in which the system wakes up and operatesin an input receiving state during which it processes the triggeringlight signals (see the table at the bottom of FIG. 8) received by thesolar cell 32 and decides the state to enter next. The falling edges ofthe voltage signal generated by the solar cell are counted in apredetermined time frame (e.g., 2 sec). The number of falling edges (orvoltage pulses) determines the state that the system enters according tothe lookup table shown in FIG. 8. Continuous non-pulsatile incidentlight of sufficient intensity causes the system to enter State 6, inwhich the battery is charged while the system is in deep sleep mode. Ifthere is continuous light impinging on the solar cell 32, but below athreshold needed or desired for charging, then the system enters State 4(deep sleep, no charging). As indicated in FIG. 8, state 5 uses a pulsecount safety range for each of the different pulse count statetransition triggers to avoid accidentally entering the wrong state.Thus, for example, 0, 1, or 2 pulse counts within a 2 second window areall treated as a 0 pulse count that sends the system into state 4 or 6depending upon the level of impinging light.

State 6, Deep Sleep State (Charging).

This state allows the system to be charged while the MCU is on and inthe deep sleep mode. The red LED may flash twice when the system entersthis state, and no LED may flashed when the system transitions back toState 4.

State 7, Erase State.

This state allows the user to erase the temperature measurement datastored in the flash memory to open up storage space for newmeasurements. The red LED may flash 6 times when this state is entered.The system returns to the deep sleep mode right after the erase isfinished, and 2 additional flashes occur to indicate this, causing theLED to flash 8 times in total.

State 8, Readout State:

This state reports data to an exterior receiver unit by flashing theLEDs to represent measured temperature data. The diagram in FIG. 9 showsthe sub-states in this state. When the system enters this state, it mayflash the red LED 9 times. During data readout, the ‘1’s may berepresented by the on state of the LED, and the ‘0’s represented by theoff state. Data may be transmitted at a rate of 16 bits/sec. For eachdata set to be sent, the MCU may first send one byte of data tocommunicate to the exterior receiver the size of the data set (number ofbytes). A ‘1’ may be appended to the front of this byte of data,signifying the start of transmission. A delay of 2 seconds may beinserted from the time that this appended ‘1’ is sent before the firstbit of the actual data is sent. Each temperature data point is codedinto two bytes; and all bytes of a data set may be sent consecutively. A‘1’ may be again appended to the front of the first byte of the data setto indicate the start of transmission. A built-in MCU resource may beused to perform CRC on all bytes of each data set, and the two-byte CRCresult may be sent at the end of the data set transmission. The systemthen may wait 2 seconds for a response from the exterior receiver.According to the response received, it may resend the same data set,send the next data set, or exit the readout state. This state may alsoend if no response is received from the exterior receiver during the 2second delay, or if all data have been sent properly.

State 9, Detection State:

This state performs the temperature measurement with a predefinedschedule. For the HPHT tests at Total, the measurement may be set to betaken every 2 minutes for up to 6 hours. This schedule can bereprogrammed by modifying the global constants in the software. Thediagram in FIG. 10 shows exemplary sub-states in this state. When thesystem enters this state, the red LED may flash 3 times. The system thenenters the sleep mode to conserve power until a measurement is to betaken. The first temperature measurement may be made 5 minutes afterentering the state to allow the user time to load the system into thetarget environment. Then the system may wake up every 2 minutes for thefollowing measurements. Between measurements, the MCU may also wake upby optical triggering (for testing or exception reasons). This cantrigger the system to exit the detection state, to reset the MCU andimmediately re-enter the detection state with the same sequence numberof temperature measurement, or to check whether the system is in thedetection state by LED flashing. If the time when the system is woken upby triggering (i.e., in State 9c) overlaps with the time that ameasurement should be taken, the measurement may be taken immediatelyafter State 9c finishes. In this state, the comparator of the MCU may bedisabled when the system detects a temperature >60° C. As explained inmore detail below, this is to avoid unexpected system wake up byinterference signals on the comparator input at higher temperatures. Thecomparator is re-enabled when a temperature below 60° C. is detected.The comparator is also re-enabled when the system finishes allmeasurements and goes to the sleep mode (State 4).

State 10, Reset State:

This state allows the user to manually reset the system should anyunexpected behavior occur, such as a clock source oscillating at anunexpected frequency. The red LED may flash 4 times when this state isentered. After reset, the system may nearly immediately enter State 4and flash the red LED another 2 times, causing the red LED to flash 6times in total.

State 11, Error State:

This is the “catch-all” state, which the system enters if the programshould fail unexpectedly and stop execution. Once the system enters thisstate, it needs to wait for the battery voltage to drop below V_(th) sothat the system power is turned off. At that point, the system goes toState 1 and the battery can be recharged.

Exemplary Operation Flow of the System

An exemplary operation flow of a field test for temperature logging withthe ELM 20 or 50 system is provided in FIG. 11. After assembly andpackaging of the system with a fresh battery, the MCU is initialized(State 3) and then put to the deep sleep mode for storage (State 4). Thesystem is expected to typically remain in State 4 until prepared fordeployment. However, if the system is stored for too long (e.g., >2weeks), the system battery may be drained below V_(th), and the MCU willbe turned off. In this case, the battery will be recharged to a voltageabove V_(th) (State 2) to allow the MCU to be re-initialized while thebattery continues to be charged (State 6). If the MCU remains on untilprepared for deployment, the battery should also be recharged to allow alonger operation lifetime (State 6). In either case, the pulse countstate (State 5) is a transient state to identify optical inputs and hasno effect on the charging. After the MCU finishes charging for apredefined period of time, it is returned to the deep sleep state (State4) to await optical triggering. When the system is ready to be deployed,the detection state (State 9) is triggered by the user by providing theappropriate pattern of light pulse input. The system performs the seriesof temperature measurements following a predefined schedule. When allmeasurements are done, the system is returned to the deep sleep state(State 4). After the system is unloaded from testing and cleaned up, theuser triggers the system into the readout state (State 8) using adifferent light pulse pattern and retrieves all measurement data in thesystem flash memory. This concludes the typical operation flow fortemperature logging.

MCU Control Program Features

The MCU software may include a number of different features to enablelow power operation at temperatures higher than the MCU specifications.These are discussed below.

Power Reduction.

In order to reduce system power consumption, the MCU may be placed inthe sleep mode as much as possible. One such example is the waiting timebetween measurements in the detection state. As another example, whenthe MCU performs a task that takes a specific amount of time (e.g., whenflashing the LEDs at a certain rate in the Readout state), the MCU maybe placed in the idle mode to further conserve power. A peripheral ofthe MCU, such as a timer, may be used to generate an interrupt at thecorrect time and wake the system from the idle mode to continue theoperation.

The clock oscillator for the system should remain on whenever the systemoperates. Its selection may be important to maintain low powerconsumption, and to keep the system operating current within the maximumcontinuous discharging current allowed by the battery. There are threeoscillators that may be integrated with the MCU: two that operate athigh frequencies (e.g., >20 MHz) for fast operation but require largecurrents that exceed the battery limitation in extended use, and onethat operates at a lower frequency (e.g., 32 kHz) and consumes much lesscurrent. The low frequency oscillator, known as the real time clock(RTC) oscillator, is therefore selected for the system operation. Thefrequency is adequate for temperature logging in the target application.

RTC Oscillator Calibration.

The low power RTC oscillator may be the only clock source available whenthe MCU is in the sleep mode. The frequency of the RTC oscillator may besensitive to temperature, increasing linearly with rising temperaturebut with inconsistent “jumps” in frequency depending upon the supplyvoltage of the MCU. Calibration of the oscillator may be necessary forthe system to wake up and take temperature measurements at the correcttime intervals. The RTC oscillator may be calibrated each time the MCUwakes up from the sleep mode by quickly comparing its frequency againsta high precision internal oscillator. The frequency of this highprecision oscillator is nearly independent of temperature, although ituses much more power and may be not suitable for the general clock.

RTC Oscillator Unintended Shutdown.

In the detection state, it was found that the RTC oscillator of the MCUhas a chance of unintended shut-down upon wake-up from the sleep mode.This may happen either at elevated temperatures or when the comparatorwakes the MCU too quickly from the sleep mode after entering it. Byincluding codes to automatically re-initialize the RTC oscillator eachtime the MCU wakes from sleep mode, this problem can be alleviated.

Interference to MCU Comparator at High Temperature.

At high temperatures encountered in the detection state, the solar celloutput voltage that is provided to the MCU comparator for opticaltriggering can have large variations that may inadvertently wake up thesystem from the sleep mode or even reset the system. Possible sourcesfor this large signal variation may be the TI nanowatt comparator usedfor the Schmitt trigger and the solar cell. Since the MCU comparator isnot required in high temperature operation, the comparator may be simplydisabled when the temperature rises above 60° C., and re-enabled afterthe temperature returns.

Implementation of Major Software Functions.

This section reviews the MCU capabilities and programming techniquesthat may be utilized to implement several major software functions,including wake-up from the sleep mode, trigger handling for stateselection, flash memory usage, and the CRC data encoding.

Sleep Mode Wake-Up.

With the MCU, options to wake up the system from the sleep mode may beport match or the comparator.

The port match technique involves giving a general purpose I/O (GPIO)pin of the MCU an expected value of ‘high’ or ‘low.’ In the case of thissystem, the pin used in port match would be connected to the positiveterminal of the solar cell and given an expected value of ‘low.’ Thesystem would wake from the sleep mode when a strong light shines on thesolar cell, causing the GPIO input to change to ‘high.’

Using the comparator for wake-up involves connecting the positive inputof the comparator to the positive terminal of the solar cell andconnecting the negative input to an internal precision voltagereference, which may have a voltage level of 1.65 V. The system can wakefrom the sleep mode when a strong light shines on the solar cell, andthe analog voltage seen by the positive input of the comparator risesabove the reference voltage.

Based on testing with an exterior receiver circuit, the comparatortechnique proved to be better than port match. The solar cell was notable to provide a strong enough signal to transition the monitored GPIOpin from low′ to ‘high’ while using port match. The comparator, however,could easily be used to transition the system out of the sleep modeusing external light. In addition, the 1.65 V voltage reference providesbuilt-in noise rejection that makes the system less sensitive to ambientlight, reducing battery power waste, and making the system more robust.

State Triggering.

When light with adequate intensity is incident on the solar cell, theMCU wakes up from the sleep mode. The light may be continuous forcharging or in a pulse pattern for optical triggering. For the latter,the number of voltage pulses generated by the solar cell during a giventimeframe (e.g., 2 sec) is counted by using the MCU comparator to detectthe falling edges of the voltage pulses. For robustness, each functionalstate is assigned with a range of acceptable numbers of pulses as listedin FIGS. 8-10. When the count number falls in a certain range, thecorresponding state is selected. An alternative approach is to use apatterned light with different combinations of ‘on’ and ‘off’ torepresent ‘1’ and ‘0’, respectively. However, processing these morecomplicated patterns other than a simple series of light pulses mayrequire more program memory.

Flash Memory Usage.

The non-volatile flash memory of the MCU may be used to storetemperature measurement data, important variables, temperature sensorcalibration constants, system status data for reset and restoring of thesystem status, etc. The chosen C8051F990 MCU divides the embedded 8 kBflash memory into pages of 512 bytes with addresses from 0x0000 to0x1FFF. General example codes from Silicon Laboratories™ showing properways to read, write, and erase the flash memory were modified for theC8051F990 MCU and used as a library when programming the systemsoftware. A set of guidelines were followed when writing and erasingflash memory in order to reduce the risk of memory corruption. Theseguidelines include enabling the power supply voltage monitor, disablingsystem interrupts during writing or erasing procedures, clearing the‘flash write enable’ and ‘erase enable’ bits after a writing or erasingoperation, and reducing the places in code where the flash memory may bewritten and erased.

Calibration constants for the offset in the temperature sensor may bestored near the highest addresses of the flash memory. The top page ofthe flash memory (addresses 0x1E00 to 0x1FFF) is the lock byte page ofthe MCU, which may not be erased using software. To easily calibratemultiple MCUs, a program may be used to automatically write the offsetdata of the temperature sensor in the flash memory page right below thelock byte page (addresses 0x1A00 to 0x1DFF). These calibration data maybe read from the flash memory during MCU initialization and stored aslocal constants to avoid unnecessary reading from the flash memory andreduce power consumption.

A page of the flash memory may be used to track the progress in thedetection state during execution to allow the system to automaticallyre-enter the detection state at the same point should a system resetoccur. These progress variables may be located below the program code onthe flash memory page with addresses from 0x0600 to 0x07FF. Thevariables include those that track the total number of bytes oftemperature measurement data stored in the flash memory (stored as a twobyte number), the number of bytes recorded for the current detectionstate (stored as a two byte number), and whether or not the comparatorshould be enabled in the detection state at the time point (stored as aone byte number). All of these values may be updated each time thesystem wakes up from the sleep mode to take a temperature measurement inthe detection state.

Storage of temperature measurement data, e.g., two bytes for each datapoint, may begin on the flash memory page immediately below the page forthe detection state variables. The first temperature data byte may bestored at address 0x05FF, and all following data may be stored atconsecutively lower addresses. When a temperature measurement is taken,the ADC may convert the temperature sensor output to a two-byte digitaldata, which forms a 16 bit union. The temperature offset constant may bethen applied to this value, and the result compared to a thresholdconstant (e.g., 60° C.) to determine if the comparator should bedisabled or enabled. The offset-compensated value may be then written tothe flash memory with the high byte at the highest address available ator below 0x05FF and the low byte stored at the address immediately belowthe high byte. The variable keeping track of the total number of databytes may be incremented twice, and the variable tracking the number ofmeasurements for the current detection state may be incremented once.All detection state variables may be then updated in the flash memory byfirst erasing the page on which they are stored and writing the newvalues. There is room in the MCU to store a total of 767 temperaturemeasurements.

The flash memory may be erased in whole-page segments by calling theerasing function. The input of the function may be any address that ispart of the page to be erased. To erase temperature measurement data,the number (n) of pages to erase may be found by dividing the variableof the total number of data bytes by the page size, and then the erasingfunction is called n times to erase n pages, starting at the address0x05FF and working toward lower addresses.

CRC Data Encoding.

The C8051F990 MCU has a built-in resource that is used to perform CRC onmultiple bytes of data by updating a CRC input register one byte at atime. The algorithm used in this MCU is described in the MCU datasheet.To encode each data set (≤40 bytes) while it is being transmitted to theexterior receiver in the readout state, the CRC result register is firstinitialized to 0x00000000. The temperature data is transmitted startingwith the high byte of the first measurement, followed by thecorresponding low byte, and then the high byte of the next measurement,etc. Each data byte is read from the flash memory, copied to the CRCinput register, and then transmitted using the LED. After alltemperature measurement data in a data set are transmitted, the highbyte of the CRC result register is read and transmitted, followed by itslow byte. Having the CRC result transmitted immediately after thetemperature data allows the exterior receiver to perform the CRCverification on the received set of data, and to request resend of thedata set immediately if necessary.

Test Results

Stress tests at temperatures ≥150° C. were performed on a prototype ofthe ELM 20 to evaluate its capability to operate at these highertemperatures. In these tests, the flexible PCBs of ELM with assembledcomponents were used without folding. Each PCB was mounted on aproto-board and tested in the setup shown in FIG. 16. During the tests,the temperature as detected by the thermocouple was increased and thenheld at the target value for 5 minutes before cooling down. Temperatureuniformity around the PCB area was checked and the variation was <2° C.

Two types of tests were performed. In Type 1 tests, the systemelectronics were powered by a 2.7 V external power supply to verifywhether the electronics work properly at the target temperatures,including 150° C. in Cycle 1 and 160° C. in Cycle 2. In Type 2 tests,the electronics were powered by a MS412FE battery, and the whole systemwas tested at 150° C. After the test, the PCBs were connected to the USBdebugger interface, and the temperature data stored in the flash memorywere retrieved.

The system successfully passed all tests. In Type 1 tests, the maximumtemperature measured by the thermocouple was 161° C. The electronicssuccessfully detected and recorded temperature data at this temperature.In Type 2 tests, the whole system including the battery successfullyoperated at 150° C. FIG. 12 shows the temperature data recorded by theMCU in one of the Type 2 tests. There was an offset of ≈4° C. betweenthe temperatures measured by the MCU and by the thermocouple, suggestingthat additional sensor calibration may be utilized at this highertemperature. In summary, the ELM system including the battery cansuccessfully operate at 150° C., and the system electronics remainsfunctional at 160° C.

ELM 50

As discussed above, FIG. 2 depicts an embodiment (ELM 50) which includesadditional sensors 74, 76 and an RFID tag 78. FIG. 13 shows a schematicfor ELM 50 that includes the ELM circuit 24 of ELM 20 with additionalpressure sensors 74 connected, as well as an inertial measurement unit(IMU) 76. The pressure sensors 74 may be implemented with a Murata™SCB10H-B250 as well as one or more other pressure sensors that can beselected for different ranges or sensitivities. The IMU may be aSTMicroelectronics™ 6-axis IMU LSM303C IMU. The RFID may be a Murata™MAGIC STRAP™ RFID Tag.

FIG. 14 depicts a state diagram for ELM 50 that includes threeadditional states than that shown in FIG. 8 for ELM 20—a battery checkstate, a parameter change state, and an inertial sensing state. Thebattery check state permits monitoring of battery voltage, which may beadvantageously done immediately prior to system deployment to ensuresufficient initial battery charge. The parameter change state permitsadjustment of the detection timing, sensor selection, and otherconfigurable settings for the system circuit operation. This may also bedone prior to deployment. FIG. 14 also provides a table of the pulsecommands that may be used by the external light source to trigger orcommand ELM 50 into the different states. The details of theseadditional three states are provided below.

State 6, Battery Check State.

In this state, the battery voltage is measured and the voltageinformation is transmitted through the optical link to the externalinterface. This allows the user to evaluate the system batterycondition, or to estimate the amount of time needed to fully charge thebattery. After being triggered into this state, the system may flash ared LED 5 times to indicate that it has successfully entered the state.The analog-to-digital converter (ADC) in the MCU then reads the batteryvoltage and converts it to digital data. This data may be opticallytransmitted to the external interface. The system returns to deep sleepstate afterwards.

State 7, Parameter Change State.

This state allows the user to modify system operation parameters such asthe measurement interval. It also allows the user to select whichpressure sensor is to be activated in the detection state, or to selectboth pressure sensors. The measurement interval determines howfrequently the system wakes up and takes measurements. Increasing themeasurement interval would save system power, but reduce the timeresolution of the measurement. The new parameters may be entered intothe Labview™ software interface by the user. The system is thentriggered into this state and the new parameters are transmitted to thesystem. The system may flash a red LED 4 times to indicate that it hassuccessfully entered this state. The system returns to the deep sleepstate after the transmission of parameters is completed.

State 10, Inertial Sensing State.

This state may be being implemented to support the inertial sensingfunction. In this state, the MCU responds to triggering signals from theIMU when motion has been detected and inertial data have been recorded.It also receives and stores the measured data in the flash memory. Thered LED may be flashed 8 times when this state is entered successfully.

FIGS. 15 and 16 show an exemplary PCB layout and tri-fold stacking ofthe components for packaging into a specified (e.g., 6.5×6.8×6.3 mm)housing space. As shown in FIG. 16, the pressure sensors may be mountedon the PCB in a manner so that the sensitive surface of the sensor isoriented away from the PCB or toward the PCB. In the latter case, anopening in the PCB may be created below the sensitive surface to permitaccurate transmission of pressure to the surface. For ELM 50, thehousing shown diagrammatically in FIG. 2 may be implemented in anymanner suitable to provide appropriate environmental protection of thecontents of the housing, while permitting optical communication into andout of the housing, and while enabling the embedded pressure sensor todetect the external pressures on the housing.

Referring now to FIGS. 17 and 18, there are shown two embodiments ofsuch a housing design. The packaging of FIG. 17 includes a rigid tubularhousing or shell, and antenna exoskeleton helically wrapped about theshell. The shell may be made from a material that is resistant toabrasion and impact. This may be filled with a transparent softpolymeric material within which the electronics and sensors are located.The soft nature of the polymer protects the electronics from impactwhile permitting the ambient pressure to be transferred to the pressuresensor in an accurate manner. The transparent nature of the epoxypermits optical communication and charging of the battery through solarcells. The filling may also include a layer of harder epoxy, as needed;this layer may be opaque to shield the electronics. The tubular rigidexoskeleton may be comprised of, or enhanced by, an antenna coil, asshown. The metal coil can be commercially acquired or a custommanufacture.

For systems that do not need the additional antenna, a package designsuch as shown in FIG. 18 could be used. This includes a metallic ornon-metallic tube without the antenna coil. A non-metallic tube may beused for systems that use radio frequency (RF) communications, while ametallic tube may be used for systems that use optical communications.

With reference now to FIGS. 19A-19D, for the packaging of ELM systems,stainless steel or alumina ceramic may be used as the tube material forthe shell of the housing. For stainless steel, the shell may be made bycutting off-the-shelf stainless steel tubes by wire EDM. For ceramicshells, custom fabrication of alumina tubes may be used.

Based on the size of the ELM system stack (e.g., 7.2×6.6×5.5 mm³),stainless steel 304 (SS304) tubing with a square cross section and aclosely matched standard size may be used. The tubing has an inner sizeof 7.8×7.8 mm² and outer size of 9.5×9.5 mm². Prototypes have been madeby cutting the stainless steel tubing into 6.5 mm lengths by wire EDM.The length tolerance was ±0.1 mm. FIGS. 19A and 19B depict the shellsmade by this cutting process. Customized alumina tubes for the ELMsystems were also designed and fabricated and are shown in FIGS. 19C and19D.

For ELM 20 and ELM 50 that utilize optical communication for receipt ofpower and commands by solar cell 32 and for transmission of data by LEDs40, a translucent polymer (polymeric material) may be used. For example,FIG. 20 depicts a completed ELM using a tubular stainless steel shelland clear silicone caulk. The caulk is used to seal into the shell anelectronic logging circuit such as shown in FIG. 3, 13, or 21 with astacked printed circuit board layout such as shown in FIG. 5-6 or 15-16.Optical charging, commanding, and readout is carried out through theclear silicone caulk. However, as noted above, some embodiments mayrequire optical shielding for one part of the system, such as bareelectronic chips, and optical transparency for the other parts, such asthe receiver and/or transmitter. Two polymeric materials may be used insuch case: one opaque and one translucent. Additionally, where apressure sensor is used, a polymeric material having a low springconstant on the order of 10⁴ N/m or less may be used to seal the sensorin the tubular shell, with the polymeric material placed over thediaphragm of the pressure sensor. This flexibility of the polymericmaterial may then be at least 10× lower in stiffness than the springconstant of the pressure sensor diaphragm. This helps reduce thetemperature coefficients of the polymer-encased pressure sensor andmitigates the impact of unit-to-unit variations in the thickness of thepolymer. Also, polymeric materials compatible with high temperatures ofat least 125° C. may be used, and preferably are able to cure at lowtemperatures to avoid degradation of the embedded battery 30. Chemicalresistance to API brine and hydrocarbons is not a major constraint, asthe packages may be covered with a polymer coating that is resistant tothese chemicals.

Depending on the application of the ELM, the Do It Best™ clear siliconecaulk (Product no. 18339 available from Do It Best Corp.) and DowCorning™ Sylgard 184 (referred to as PDMS) may be used as thetranslucent polymer. For applications using an opaque polymer for someor all of the shell filling, DOW Corning 736 heat resistant siliconecaulk may be used. Any other suitable polymeric material may be used aswell and will depend upon the particular environmental application withwhich the ELM is used.

Both the caulk and the PDMS belong to the silicone category of polymericmaterials. Both are soft, clear, and serviceable over 200° C. Both canbe cured at room temperature. Their properties are listed in Table 1.The chemical composition of the caulk is mainly hydroxyl-terminateddimethyl siloxane, and of the PDMS is dimethylvinylsiloxy-terminateddimethyl siloxane. The different end functional groups result indifferences in the properties of the polymers. The caulk has goodresistance to brine and moisture, and has good adhesion to manymaterials. The PDMS does not chemically degrade in brine; however, itreadily absorbs moisture. It also has very low viscosity before curing,which is desirable for filling voids in the system stack. Additionally,cured PDMS has excellent transparency, which is desirable for opticalcommunication, compared to the caulk.

TABLE 1 Product Do It Best ™ Silicone Dow Corning ™ Sylgard Sealant(“caulk”) 184 (“PDMS”) Main composition Dimethyl siloxane, Dimethylsiloxane, hydroxyl-terminated dimethylvinylsiloxy- terminated Springconstant ≈2 × 10⁴ N/m ≈2 × 10⁴ N/m Color Translucent Transparent Servicetemp. −50° C.-230° C. −45° C.-200° C. Curing temp. 25° C. 25° C.-150° C.Viscosity before ≈50 (toothpaste-like) ≈3.5 (honey-like) curingCompatibility Compatible Compatible with brine

FIG. 21 depicts a third embodiment of an electronic logging circuit thatmay be constructed the same or similar to that of the logging circuitsof ELM 20 and ELM 50. Thus, the various aspects of the construction andoperation of ELM 20 and ELM 50 discussed above may be utilized for thethird embodiment of the electronic logging circuit as well, except wherespecific differences or variations are described below. One suchdifference is that the third embodiment of the electronic loggingcircuit includes a switch in the charging circuit that permits the ELMsystem to be fully shut down to preserve the battery life during storageof the system before deployment and/or after retrieval. The switch maybe a mechanical switch such as a slide switch that uses a sliding knobfor activation. Alternatively, it can be a switch that is activated byRF, magnetic, acoustic, optical, or thermal means. Suitable mechanical,electrical, thermal, and magnetic switches and circuits for operatingthem will be known to those skilled in the art.

Another difference is that the third embodiment of the electroniclogging circuit uses reference and series capacitors for improvedoperation. The reference capacitor is included to allow datacompensation for the temperature effect of the capacitance-to-digital(CDC) converter in the MCU. This permits the third embodiment of theelectronic logging circuit to determine how the circuit characteristicsdrift with temperature, enabling the processor to compensate for thesedrifts. For example, leakage current in the pressure sensors and/orother components may change with temperature, and the referencecapacitor permits the processor to properly compensate for these changesusing suitable calculations that will be known or apparent to thoseskilled in the art. Those calculations can be incorporated into the ELMsoftware. The reference capacitor used may be selected to have a lowtemperature coefficient and low pressure response, and may be, forexample, a ceramic capacitor having a similar capacitance (e.g., a fewpicofarads) that is the same as or similar to that of the pressuresensor(s) used.

The third embodiment of the electronic logging circuit also includes aseries capacitor between each of the ECU inputs and the correspondingsensor to protect the ECU from an inadvertent short circuit to ground ifthe sensor fails. These series capacitors may have a capacitance valuemuch larger than the capacitance of the pressure sensor.

The non-volatile flash memory of the MCU is used to store temperatureand pressure measurement data, system variables, temperature sensorcalibration constants, system status data for system reset, etc. Inorder to reduce the risk of memory corruption, a set of guidelines maybe utilized when writing and erasing the flash memory. These guidelinesinclude enabling the power supply voltage monitor, disabling systeminterrupts during writing or erasing, clearing the ‘flash write enable’and ‘erase enable’ bits after a writing or erasing operation, andreducing the places in the program code where the flash memory may bewritten and erased. Example codes from Silicon Laboratories for reading,writing, and erasing the flash memory of the C8051F990 MCU may be usedas standard functions when programming the system software. Also, thesesoftware codes may be modified as appropriate for a particularimplementation.

Other improvements may also be made in flash memory-related operationsto reduce power consumption. The 8 kB flash memory embedded in theC8051F990 MCU is divided into pages of 512 bytes with addresses from0x0000 to 0x1FFF. The operations of the flash memory, i.e., reading,writing or erasing, may be always carried out on a whole page basis,even if only a single byte of data is needed. In other embodiments,2-byte measurement data was written to the flash memory after eachmeasurement. In the third embodiment of the electronic logging circuit,to reduce the number of flash memory operations and save power, the datais temporarily stored in the RAM after each measurement. A variabletracking the total number of bytes of measured data is stored in theflash memory with other system status data. This variable is updatedeach time the system exits the detection state. The data in the RAM iswritten to the flash memory every time when the variable reaches themultiple of a predetermined value. The battery voltage may also bechecked after each measurement. The data in the RAM may also be writtento the flash memory when the battery voltage is lower than a thresholdvalue.

The constants for calibration of the temperature sensor are stored in aflash memory page. In other embodiments, these constants may be readform the flash every time a measurement is taken. To eliminateunnecessary reading operations from the flash memory and reduce powerconsumption, the calibration data of the third embodiment of theelectronic logging circuit are now read from the flash memory onceduring MCU initialization and stored as local constants in the RAM.

The C8051F990 MCU has 8 kB Flash memory space for both the assembly codeof the program and the measurement data. With the aforementioned newfeatures, the program size may increase. This reduces the memory spaceavailable for data storage, despite more types of measurement data to bestored. To address this issue, the data storage format may be rearrangedso that the memory space can be used more efficiently. In otherembodiments where memory space is more abundant, each temperaturemeasurement may have a 10-bit resolution, and 2 bytes (16 bits) ofmemory space may be used for that. Six bits of the space is thus wastedfor each measurement point. In the third embodiment of the electroniclogging circuit, when there is one pressure sensor activated, 10-bittemperature data is combined with 14-bit pressure data to form 3 byte ofdata, instead of the 4 bytes that would be necessary to storetemperature and pressure data separately, saving 1 byte for eachmeasurement or allowing 33% more measurements to be stored. When bothpressure sensors (or the reference and one pressure sensor) areselected, the 10-bit temperature data is followed by two 11-bit (lowresolution, or LR mode) or two 14-bit (high resolution, or HR mode)segments of pressure data. In this case, each measurement occupies 4bytes (LR mode) or 5 bytes (HR mode) of flash memory instead of 6 bytes,saving up to 2 bytes for each measurement or allowing up to 50% moremeasurements to be stored. Table 2 shows the calculations of the numberof measurements (data sets) that can be stored in the MCU memory usingthe new scheme. Assuming that the measurement is taken every 2 min. and3.5 kB out of the total 8 kB is available for data storage, up to 40hours of data can be stored for measurement of both temperature andpressure.

TABLE 2 One P One P Two P Two P Three P data data data (LR) data (HR)data (HR) or one and one and one and one and one T data T data T data Tdata T data Pressure 14 14 2x 11 2x 14 3x 14 bits bits bits bits bitsTemperature 10 10 10 10 10 bits bits bits bits bits Total bytes   2   3 4  5  7 Total data sets 1792 1194 896 716 512 Max time* >59 h 39 h 29 h23 h 17 h 48 min 52 min 52 min 4 min

Referring finally to FIG. 22, there is shown another packagingembodiment in which the ELM is maintained in a flat (unfolded) conditionfor uses in applications either requiring or benefiting from a lowprofile ELM. A printed circuit board layout that is the same as orsimilar to that shown in FIGS. 5-6 or FIGS. 15-16 can be used. The flatELM of this embodiment may be encapsulated in a protective polymer orotherwise attached to or incorporated into a rigid or flexible carrier.

Advantageous Features

Those skilled in the art will appreciate the following unique featuresof the ELM designs described above.

-   -   The ELM system functional states for charging, sleep, wake-up,        optical communication, detection, data readout, etc., and the        transitions between the states with optical triggering. (see        FIG. 8)    -   The approach used to serially transmit data with or without CRC        error check through an optical link with an external readout        unit. (see FIG. 9)    -   The approach used to wake up the system at a preset time period        to perform a series of measurements of temperature (and other        environmental parameters), while allowing optical triggering to        interact with the system during the waiting time between the        measurements. (see FIG. 10)    -   The operation flow of the ELM system during a field measurement.        (see FIG. 11)    -   The techniques used to maximize reduction in system power        consumption:        -   The MCU enters the deep sleep mode during all waiting time            periods during which no active task is to be performed, and            using the low power options for system resources (e.g., the            lower power oscillator).        -   The MCU enters the idle mode during active tasks of extended            duration (e.g., flashing LEDs at a certain rate for data            readout). It is then triggered back into the active mode by            an interrupt from an MCU peripheral such as a timer to            continue the operation.    -   The techniques used to allow the system components to operate        properly at high temperature:        -   The internal oscillator that is used for its low power            consumption can have high sensitivity to temperature and            supply voltage. Each time the MCU wakes up, this oscillator            is calibrated quickly against a high precision internal            oscillator that consumes more power. This method allows            accurate timing to be maintained for system operation in the            high temperature environment.        -   The MCU comparator used for optical triggering is disabled            at high temperatures to prevent unintended triggering of the            system, as might be caused by large variations in the input            voltage of the comparator at these temperatures. The            comparator is re-enabled when the measured temperature drops            below a certain threshold so that the capability for optical            communication may be restored.

The invention claimed is:
 1. An environmental logging system comprising:a housing; an electronic logging circuit carried by the housing, thelogging circuit including: an energy source, transducer, and chargingcircuit interconnecting the energy source and transducer, wherein theenergy source is charged by the charging circuit using electricity fromthe transducer in response to external energy applied to the transducer;an electronic control unit (ECU) that includes a processor, memory, andone or more sensors, wherein the ECU operates under power from theenergy source to store data from the sensor(s); and a transmittercoupled to the ECU, wherein the transmitter transmits electromagneticradiation outside the housing; wherein the ECU is coupled to thetransducer and is configured to detect electricity from the transducerthat is indicative of pulses of external energy impinging on thetransducer, the ECU being configured to respond to one or more commandsencoded by the pulses to change operating states between a sleep state,detection state, and readout state; wherein the ECU is configured torecord data from the one or more sensors when in the detection state andto send data via the transmitter when in the readout state.
 2. Theenvironmental logging system set forth in claim 1, wherein the ECUcomprises an integrated circuit that includes the processor and memory,wherein the integrated circuit has at least one output coupled to thetransmitter to output the data to the transmitter for transmission ofthe data via the electromagnetic radiation, and has an input coupled tothe transducer to receive pulses of electricity indicative of the pulsesof external energy impinging on the transducer, whereby the transducerprovides charging current to the energy source and operating commands tothe processor.
 3. The environmental logging system set forth in claim 1,wherein the transducer receives the external energy via optical or radiofrequency communication, and the transmitter transmits the data viaoptical or radio frequency communication.
 4. The environmental loggingsystem set forth in claim 1, wherein the logging circuit is sealedwithin the housing, with the transducer being located in the housingsuch that it receives the external energy through a portion of thehousing that is transmissive to the external energy, and wherein thetransmitter is positioned within the housing such that electromagneticradiation from the transmitter is emitted through the housing.
 5. Theenvironmental logging system set forth in claim 4, wherein the housingcomprises a tubular shell filled with a polymeric material that istransparent to light and that seals the logging circuit within theshell, and wherein the transducer receives the external energy as lightentering the housing through the polymeric material.
 6. Theenvironmental logging system set forth in claim 1, further comprising aprogram stored in the memory that, upon execution by the processor,monitors for receipt of the one or more commands and in response toreceiving a specific command, switches into one of a plurality ofoperating states, wherein the plurality of operating states include thesleep state, the detection state, the readout state, an erase state, areset state, and an error state.
 7. The environmental logging system setforth in claim 6, wherein the program, upon execution by the processor,responds to receipt by the transducer of the external energy at a levelabove a threshold to switch to an input receiving state during which theprocessor monitors for the receipt of the one or more commands andswitches from the input receiving state to one of the other plurality ofoperating states.
 8. The environmental logging system set forth in claim1, wherein the logging circuit further includes: a trigger circuit thatoperates from the electricity supplied by the energy source and that iscoupled to the ECU; wherein the energy source provides the electricityat different voltage levels depending on a state of charge of the energysource and wherein the processor is rated for normal operation atvoltages above a minimum operating voltage and draws more power at somevoltages under the minimum operating voltage; and wherein the triggercircuit inhibits power draw by the processor at voltages under theminimum operating voltage.
 9. The environmental logging system set forthin claim 8, wherein the charging circuit includes a power output and theprocessor includes a power input, and wherein the trigger circuit isconnected between the power output of the charging circuit and the powerinput of the processor and is configured to supply operating power tothe processor via the power input when the voltage level of the energysource is at a threshold voltage that is above the minimum operatingvoltage.
 10. The environmental logging system set forth in claim 9,wherein the trigger circuit comprises a comparator that receives powerfrom the energy source and that includes inverting and non-invertinginputs and an output, with the non-inverting input receiving an inputsignal at a voltage dependent on the voltage of the energy source, theinverting input receiving a reference voltage, and the output beingcoupled to the non-inverting input to provide positive feedback, andwherein the output of the comparator is coupled to the power input ofthe processor to provide the operating power to the processor.
 11. Theenvironmental logging system set forth in claim 9, wherein the triggercircuit is configured to switch on power to the processor via the powerinput when the voltage level of the energy source increases past a firsthysteresis threshold voltage and to switch off power to the processorvia the power input when the voltage level of the energy sourcedecreases below a second hysteresis threshold voltage that is lower thanthe first hysteresis voltage level, wherein the first and secondhysteresis voltage levels are greater than the minimum operating voltageof the processor.
 12. An environmental logging system comprising: ahousing; an electronic logging circuit carried by the housing, thelogging circuit including: an energy source, transducer, and chargingcircuit interconnecting the energy source and transducer, wherein theenergy source is charged by the charging circuit using electricity fromthe transducer in response to external energy applied to the transducer;an electronic control unit (ECU) that includes a processor, memory, andone or more sensors, wherein the ECU operates under power from theenergy source to store data from the sensor(s); wherein the housingcomprises a tubular shell filled with a polymeric material that sealsthe logging circuit within the shell, wherein the logging circuitfurther comprises a pressure sensor sealed within the polymericmaterial, wherein the polymeric material permits ambient pressureoutside the housing to be transferred to the pressure sensor.
 13. Theenvironmental logging system set forth in claim 12, wherein the tubularshell is sealed by the polymeric material at an end of the shell withthe transducer being located within the polymeric material such that itreceives the external energy in the form of light received through thepolymeric material as light from an external light source, and whereinthe logging circuit further comprises a transmitter positioned in thepolymeric material such that the transmitter transmits light pulsescontaining data out of the housing through the polymeric material. 14.The environmental logging system set forth in claim 12, furthercomprising a transmitter that transmits data from the processor to anexternal receiver, wherein the logging circuit is configured to useradio frequency communication for sending and/or receiving power, data,or both.
 15. The environmental logging system set forth in claim 12,wherein a spring constant of the polymeric material is on an order ofabout 2×10⁴ N/m or less.
 16. The environmental logging system set forthin claim 12, wherein a spring constant of the polymeric material is atleast ten times lower than a spring constant of a diaphragm of thepressure sensor.
 17. An environmental logging system comprising: ahousing; an electronic logging circuit carried by the housing, thelogging circuit including: an energy source, transducer, and chargingcircuit interconnecting the energy source and transducer, wherein theenergy source is charged by the charging circuit using electricity fromthe transducer in response to external energy applied to the transducer;an electronic control unit (ECU) that includes a processor, memory, andone or more sensors, wherein the ECU operates under power from theenergy source to store data from the sensor(s); wherein the housingcomprises a tubular shell filled with a polymeric material that sealsthe logging circuit within the shell, wherein the logging circuitfurther includes: a trigger circuit that operates from the electricitysupplied by the energy source and that is coupled to the ECU; whereinthe energy source provides the electricity at different voltage levelsdepending on a state of charge of the energy source and wherein theprocessor is rated for normal operation at voltages above a minimumoperating voltage and draws more power at some voltages under theminimum operating voltage; and wherein the trigger circuit inhibitspower draw by the processor at voltages under the minimum operatingvoltage.
 18. The environmental logging system set forth in claim 17,wherein the charging circuit includes a power output and the processorincludes a power input, and wherein the trigger circuit is connectedbetween the power output of the charging circuit and the power input ofthe processor and is configured to supply operating power to theprocessor via the power input when the voltage level of the energysource is at a threshold voltage that is above the minimum operatingvoltage.
 19. The environmental logging system set forth in claim 18,wherein the trigger circuit comprises a comparator that receives powerfrom the energy source and that includes inverting and non-invertinginputs and an output, with the non-inverting input receiving an inputsignal at a voltage dependent on the voltage of the energy source, theinverting input receiving a reference voltage, and the output beingcoupled to the non-inverting input to provide positive feedback, andwherein the output of the comparator is coupled to the power input ofthe processor to provide the operating power to the processor.
 20. Theenvironmental logging system set forth in claim 19, wherein thecomparator is a nanowatt comparator having an internal voltage referenceconnected to the inverting input to supply the reference voltage to theinverting input.
 21. The environmental logging system set forth in claim18, wherein the trigger circuit is configured to switch on power to theprocessor via the power input when the voltage level of the energysource increases past a first hysteresis threshold voltage and to switchoff power to the processor via the power input when the voltage level ofthe energy source decreases below a second hysteresis threshold voltagethat is lower than the first hysteresis voltage level, wherein the firstand second hysteresis voltage levels are greater than the minimumoperating voltage of the processor.